A modular multilevel converter (hereinafter referred to as an MMC) is formed by connecting a plurality of cells each including a switching element, capable of on-off control and a DC capacitor to constitute an arm, and each cell in the arm outputs voltage pulses of different phases to synthesize a multilevel waveform with less harmonics as the whole arm. In this MMC, since a capacitor voltage in each cell can be evenly divided by control, it is possible to increase the voltage by adding cells, so that application to a large capacity power system converter is being advanced.
In this MMC, it is proposed to provide a bypass circuit in parallel to the cell so that operation can be continued even when the cell fails. A bypass circuit is provided for each cell, and when the cell fails, the failed cell is specified and a bypass circuit, provided in the cell is short-circuited so that the failed cell is not involved in the output. The MMC using a short-circuit switch as the bypass circuit is proposed, for example, in Patent Document. 1.